Densitometer

ABSTRACT

A densitometer which utilizes a thin vane vibrated by a magnetostrictive tube. A piezoelectric crystal pickup feeds an amplifier and tracking filter to energize a coil around the magnetostrictive tube. The device is, in effect, an electromagnetic oscillator in that the coil is driven in phase with the detected signal. Vibration thus increases in amplitude until limited electrically. A linearization circuit provides an output D.C. voltage directly proportional to fluid density, gas, or liquid, which may be impressed upon a conventional D.C. voltmeter calibrated linearly in density. A great many features, both in the mechanical structure and in the electronics, make it possible to easily calibrate the instrument to read, for example, to within an accuracy tolerance of + OR - 0.1 percent over a fluid density range of, for example, from about 0.08 pounds per cubic foot to 80.0 pounds per cubic foot. Another outstanding advantage of the invention relates to a probe type construction which may be used in a pipeline of any size.

United States Patent 51 July 18, 1972 Miller et al.

[ DENSITOMETER 129,357 6/1959 U.S.S.R. ..73/l94B [72] Inventors: Charles Evelelgh Miller; Gerald Lance Prim), Examiner Richard cloueisser Schlatter, both of Boulder, Colo.

Assistant Examiner-C. E. Snee, lll AttorneyC. Cornell Remsen, Jr., Walter J. Baum, Paul W.

[73] Assignee: International Telephone and Telegraph Corporation, New York, NY. Hemminger, Thomas E. Kristofferson and Charles L. Johnson,

[22] Filed: Aug. 20, 1970 21 Appl. No.: 65,371 ABSTRACT A densitometer which utilizes a thin vane vibrated by a mag- [52] US. Cl ..73/32, 73/194 B netostrictive tube- A pi z c ric cry l pickup feeds an am- [51] Int. Cl. ..G0ln 9/00 p ifi r n k ng filter to en rgize a coil around the mag- [58] Field or Search ..73/30, 32, 67, 67.1, 67.2, netostrictive tube- The device in f t, an agn tic 73/194; 310/26; 318/118; 340/10, 11; 285/95, 99, oscillator in that the coil is driven in phase with the detected 158, 161, 347, 343 signal. Vibration thus increases in amplitude until limited electrically. A linearization circuit provides an output D.C, volt- [56] References Cited age directly proportional to fluid density, gas, or liquid, which may be impressed upon a conventional D.C. voltmeter UNITED ST PATENTS calibrated linearly in density. A great many features, both in the mechanical structure and in the electronics, make it possiwlegand et "285/347 ble to easily calibrate the instrument to read, for example, to 3254284 5/1966 within an accuracy tolerance of 10.1 percent over a fluid den- 3282084 1H9 sity range of, for example, from about 0.08 pounds per cubic 3426593 2/l969 foot to 80.0 pounds per cubic foot. Another outstanding advantage of the invention relates to a probe type construction FOREIGN PATENTS OR APPLICATIONS which y be used in a p p of y sizel,287,559 11/1962 France ..73/32 30 Claims, 11 Drawing Figures 1 i all "1 a a "'lE1111 7 Nmlfike W 56 I: 2 i I is a -F\\\\\w.\\\\- i l 55 6/ es ii 4? \i i 52 i w 6/ we, wil 54 6o S V 1 V M 42 \V/ a 48 %a 40 3 er A 44 Q7 /8 11H 11 "1mg 4/ |||I i11 25 Q J8 i 3h DENSITOMETER BACKGROUND OF THE INVENTION This invention relates to instruments for producing output signals as a function of the density of a fluid, and more particularly, to a vibration densitometer.

In the prior art, densitometers have been developed in which structures have been submerged in a fluid and vibrated. The frequency of vibration of the structure was then a function of the density of the fluid. In all such prior art densitometers, either the entire flow of a fluid in a pipeline was directed through the densitometer, or a bypass was constructed to divert a portion of the fluid through the densitometer. Both of these arrangements of the prior art have had several serious disadvantages. In the first place, those instruments taking the entire pipeline flow could not be used where the pipeline was even moderately large. This was true because it was impractical to make a densitometer fairly large. It was also impractical to make densitometers in a great many sizes to fit pipelines of all sizes.

In the prior art instruments utilizing the bypass, pressure differentials and changes due to the bypass connection have caused large errors in density measurement.

In addition to the foregoing, vibration densitometers of the prior art have required the use of a great many expensive and complicated component parts.

SUMMARY OF THE INVENTION In accordance with the device of the present invention, the above-described and other disadvantages of the prior art are overcome by providing a vibration densitometer with a probe for insertion into a fluid. In accordance with the present invention, the probe may thus be inserted into the fluid in any manner. However, the output of the densitometer of the present invention may often be used in the measurement and indication of the density of a gas or liquid in a pipeline. Moreover, the output of the densitometer of the present invention may also be used in combination with a volume rate of flow signal to produce an indication of the rate of mass flow or the total mass flow through a pipeline.

Since the densitometer of the present invention has an immersible probe, the probe may thus be simply mounted through a suitable opening in the wall of a pipeline. The inside diameter of the pipeline thus does not restrict the use of the device of the invention. In other words, it may be used in a pipeline of any size. Further, the device of the invention is extremely accurate and, in addition, is not subjected to the pressure differentials and changes attendant upon the use of the prior art bypass. For example, the densitometer of the present invention may be easily calibrated, for example, to within an accuracy tolerance of :t 0.1 percent over an extremely wide range of densities. That is, the wide range is considerably larger than that of any range of any known prior art densitometer. For example, the densitometer of the present invention may be calibrated to within the said 10.] percent accuracy over a range from about 0.08 pounds per cubic foot to about 80.0 pounds per cubic foot.

Notwithstanding the foregoing outstanding features of the present invention, the present invention requires only the use of relatively few inexpensive and uncomplicated component parts. For example, the vibrating structure used in accordance with the present invention may simply be a vane. For example, the vane may be a thin membrane having also parallel flat surfaces. The vane may be rectangular and have external surfaces defining a right parallelopiped Preferably, the vane is made of any conventional metal which will not erode with use. For example, the vane may be made of stainless steel.

In addition to the foregoing, the densitometer of the present invention has a great many other advantages, may of which are also outstanding. Some of these advantages are outlined hereinafter. For example, there are many mechanical arrangements in the probe which make the use of complex component parts unnecessary. The use of a piezoelectric crystal pickup also makes it possible to reduce the size of the probe considerably.

Another advantage of the invention relating to approved efficiency and accuracy relates to a structure for clamping the edges of the vanes with a pressure of, for example, 20,000 pounds per square inch.

Many component parts of the densitometer of the present invention, and combinations thereof, perform two or more functions simultaneously. One example of such a combination is the use of an outer magnetic tube and an inner magnetostrictive tube which form, more or less, the body of the shank of the probe. The use of magnetic materials in both inner and outer tubes makes it possible to shield electrical leads to a driver coil between the tubes and leads from the crystal through the center of the inner tube. That is, the leads from the driver coil are maintained between tubes, and the leads from the crystal are positioned inside the inner tube. This prevents currents from being induced in the crystal circuit by the current in the driver coil leads. The signal on the crystal leads is amplified before all four leads are unshielded.

A further advantage of the present invention is the construction of the inner and outer tubes to form a substantially completely closed magnetic circuit around the driver coil which is located in a space between e tubes. This makes the driver coil magnetic coupling with the inner tube a maximum. The magnetostrictive inner tube, which is employed to vibrate the vane, is thus driven with a maximum efficiency.

Another advantage of the present invention resides in the use of an outer tube construction which holds at least a portion of the inner tube in axial compression against a driver member at all times. That is, the inner tube is in axial compression both when the driver coil is energized and when the driver coil is deenergized. The degree of axial compression thus changes when an alternating current is passed through the driver coil. The efficiency of vibration transmitted from the inner tube is thus increased. There is no wasted motion, and there is no energy loss due to hammer. That is, the inner tube does not hammer any structure.

Another advantage of the invention resides in the use of a resilient mounting for connection between the probe and, for example, a pipeline.

Another feature of the invention resides in the use of a resilient seal for the probe which performs at least two functions. The seal first performs the resilient mounting function previously described, and it also seals off the fluid and prevents a leak thereof from between the probe of the opening in the pipeline in which the probe is inserted. The resilient mount of the probe substantially increases the efficiency and accuracy of the instrument.

Efficiency and accuracy of the densitometer of the present invention is also increased by the use of a resilient mount for an electrical connector otherwise substantially fixed relative to the probe.

Another feature'of the invention resides in means for impressing a signal upon the driver coil responsive to the output signal of the crystal. The driver coil is supplied with a signal in phase with the output of the crystal. The densitometer of the present invention thus acts, in part, as a electromagnetic oscillator. If desired, the oscillator can be prevented from running away by limiting the output signal magnitude of the means employed to impress a signal on the driver coil.

The means for controlling the driver coil may include a differentiator and a tracking filter. It is an advantage of the invention that the differentiator can attenuate the lower frequencies while applying a phase shift at at all frequencies. Thus, the tracking filter likewise may be connected to provide a phase shift over 90 in the same direction, and the driver coil signal may be set in phase merely by connecting the driver coil leads with the proper polarity.

Another feature of the invention resides in the use of a highly accurate analog conversion apparatus for taking a signal of one frequency and converting it to another function of frequency. By the use of this device, it is possible to obtain a D.C. output voltage directly proportional to the density of the fluid being metered. Thus, a D.C. voltmeter calibrated linearly in density may be employed as an indicator.

Another feature of the present invention resides in the use of an extremely simple method of calibrating the densitometer to within the said i 0.01 percent accuracy over the said wide range.

For purpose definition herein and in the claims, the phrase first resonant frequency hereby is defined to mean the lowest frequency at which the said electromagnetic oscillator will oscillate. Note will be taken that depending upon the location and band width of the tracking filter passband, the said electromagnetic oscillator may oscillate at any one of several resonant frequencies. It is also to be pointed out that whichever resonant frequency is selected, the resonant frequency changes with density and it is this change in resonant frequency that produces an indication of the density of a fluid.

Another feature of the invention resides in the said resilient probe mounting. This resilient probe mounting keeps the said electromagnetic oscillator from jumping from one resonant frequency to another without any outside stimulus.

As will be explained, the lowest resonant frequency is preferred because, in general, the largest signal-to-noise ratio may be obtained for this condition.

The word densitometer is hereby defined for use herein and in the claims to include an instrument without, as well as with, a density or other indicator. For example, the device of the present invention can produce an output signal which is a D.C. voltage directly proportional to density. Thus, if the invention were combined into a mass rate of flow meter, the indicator would indicate the mass rate of flow, and not density. It would thus be unnecessary to have an indicator to indicate density. The said D.C. analog voltage would then only be used in a multiplier to derive a signal directly proportional to, for example, the product of the density and volume flow rate analogs. The output signal magnitude would then be directly proportional to mass rate of flow. The magnitude of a mass rate of flow analog could be displayed on an indicator. Alternatively, a total mass flow indicator could be used by integrating the multiplier output. A density or mass rate of flow or other analog could be used in controlling a process. In this case, no indicator whatsoever would be needed.

The word differentiator, as used herein, includes an amplifier. This differentiator is entirely conventional, but is not identical to other conventional differentiators of the prior art. Thus, the word differentiator is defined for use herein and in the claims to include the differentiator shown in the drawings and described herein and any equivalent thereof.

From the foregoing, it will be appreciated that the densitometer of the present invention may be used in a pipe or another container of a fluid tight construction. However, the pipe or other container need not be fluid tight. A simple fluid receptacle will do as well. Thus, the probe of the invention may simply be submerged in a fluid. Further, it is a feature of the invention that the probe may be inserted either into a gas or a liquid, or both in succession, and the density of the gas or the density of the liquid or the densities of both may be determined even without a change in calibration.

The above-described and other advantages of the present invention will be better understood from the following detailed description when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings which are to be regarded as merely illustrative:

' FIG. 1 is a perspective view of a densitometer probe constructed in accordance with the present invention;

FIG. 2 is a sectional view of the probe taken on the line 22 shown in FIG. 1;

FIG. 3 is a perspective view of a group of component parts of the probe shown in FIG. 1;

FIG. 4 is a transverse sectional view of the assembly taken on the line 44 shown in FIG. 3;

FIG. 5 is an enlarged longitudinal sectional view of a portion of the probe shown in FIG. 1;

FIG. 6 is a longitudinal sectional view of a portion of mounting means for an electrical connector otherwise substantially fixed relative to the probe taken on the line 6-6 shown in FIG. 2;

FIG. 7 is a block diagram of a densitometer constructed in accordance with the present invention;

FIG. 8 is a schematic diagram of a current-to-voltage converter shown in FIG. 7;

FIG. 9 is a schematic diagram of a differentiator shown in FIG. 7;

FIG. 10 is a schematic diagram of two of the blocks shown in FIG. 7, two other of the blocks also being shown in relation thereto; and FIG. 11 is a schematic diagram of a linearization circuit shown in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, the probe of the present invention is indicated at 10 having a shank 11, a housing 12 at its upper end, a tubular assembly 13 at its lower end, and an electrical connector assembly 14 at the upper end of housing 12 fixed thereto by bolts 15. Annular fittings I6 and 17 extend around shank 11 for mounting probe 10 in a hollow cylindrical extension 18 of a pipeline 19, as shown in FIG. 2.

As shown in FIGS. 1 and 2, a stainless steel vane 20 is mounted in assembly 13 in a position perpendicular to the axis of a hollow cylindrical magnetostrictive inner tube 21. Vane 20, if desired, may be also mounted in a symmetrical position with respect to the axis of an outer sleeve 22 which houses it.

Vane 20 may be a rectangular plate having flat and parallel upper and lower surfaces as shown in FIG. 2, and may otherwise have mutually normal surfaces forming a right parallelopiped.

Shank 11 not only includes inner tube 21, but an outer magnetic tube 23. A driver coil or solenoid winding 24 wound on a nylon bobbin 25 is press fit onto the external surface of inner tube 21 and located in a space between the tubes 21 and 23 toward the lower end of shank ll. Coil 24 is thus maintained in a substantially fixed position on inner tube 21, although the same is not necessarily critical to the operation of the device of the present invention.

Vane 20 is supported between two half cylinders 26 and 27 as shown in FIGS. 2 and 3. According to the invention, the longitudinal edges of vane 20 are pressed together between half cylinders 26 and 27 with a pressure of, for example, 20,000 pounds per square inch because the assembly shown in FIG. 3 is inserted in sleeve 22 with an interference fit, sleeve 22 being heated prior to the said insertion.

Half cylinder 26 has four projections 28, and half cylinder 27 has four projections 29. Projections 28 and 29 serve to prevent longitudinal movement of vane 20 between half cylinder 26 and half cylinder 27 although the same is not likely due to the clamping pressure on vane 20 between half cylinder 26 and half cylinder 27.

Half cylinders 26 and 27, and vane 20 may be machined to have a flat or recess to receive a piezoelectric crystal 30. Crystal 30 has electrical leads 31 and 32 which extend around half cylinders 26 and 27 in grooves 33 and 34, respectively, to a point where they enter the hollow interior of inner tube 21. This entry is made at the lower end of innertube 21, as shown in FIG. 2.

As shown in FIG. 3, projections 28 and 29 may have a slight separation at 35 to insure that the pressure contact of half cylinders 26 and 27 on vane 20 is quite high due to the said interference fit.

As shown in FIG. 2, a boss 36 is welded at 37 to sleeve 13 in a fluid tight manner. Although the device of the present invention need not always be fluid tight throughout, a glass-to-metal seal or other seal may be provided inside inner tube 21 for leads 31 and 32. Before the said interference fit is provided, if

desired, crystals 30, and those portions of leads 31 and 32 in grooves 33 and 34, respectively, may be potted with an epoxy. Further, after the interference fit has been effected, the entire unit when completely assembled may be treated further by applying a bonding agent around all of the structures inside sleeve 22. Any conventional bonding process may be employed including, but not limited to, the application of a bonding agent sold under the name of Locktile."

As stated previously, boss 36 may be welded to sleeve 22 at 37 in a fluid tight manner. Further, outer tube 23 may be threaded onto boss 36 and welded thereto at 38 in a fluid tight manner. For all practical purposes, boss 36 may thus be considered an integral part of outer tube 23. Boss 36, for example, is also made of a magnetic material. All of the magnetic materials referred to herein may be any magnetic material including, but not limited to, stainless steel. However, inner tube 21, although being magnetic, must also be magnetostrictive. Notwithstanding this limitation, it is to be noted that inner tube 21 is employed to produce vibration, and if one feature of the present invention is used without another, the use of a magnetostrictive or magnetic material may not be required, and the invention still practiced.

Inner tube 21 has an annular projection 39 with a shoulder 40. Outer tube 23 has a lower bore 41 separated from a smaller upper counter bore 42 by an annular shoulder 43. Shoulder 40 and 43 abut. From shoulder 40 to the lower end of inner tube 21, inner tube 21 is always in axial compression. That is, inner tube 21 is in compression when coil 24 is energized, but inner tube 21 is also in compression when coil 24 is deenergized. Coil 24 is energized with an alternating current which thus merely changes the degree of compression of inner tube 21.

Projection 39 has a hole 44 through which the electrical leads of coil 24 can pass from the location of coil 24 upwardly between tubes 21 and 23.

The manner in which probe is mounted in pipeline 19 is better illustrated in FIG. 5. In FIG. 5, note will be taken that outer tube 23 has an outwardly extending radial projection 45 on each side of which rubber O-rings 46 and 47 are compressed by fittings 16 and 17. Fitting 17 is threaded into extension 18 and sealed thereto by a conventional sealing compound 48 shown in FIG. 2. In FIG. 5, note will be taken that fitting 16 is threaded inside fitting 17 at 49. The amount 0- rings 46 and 47 are compressed is, therefore, determined by the position of fitting 16. That is, fitting 16 is turned, for example, by a wrench, until the desired O-ring compression is reached.

From the construction illustrated in FIG. 5, note will be taken that only O-rings 46 and 47 contact outer tube 23, and that, therefore, shank 11 is never touched by either fitting 16 or fitting 17.

It is an advantage of the present invention that the construction of probe 10 is such that the leads from coil 24 are kept magnetically separate from the leads from crystal 30. This is true through a portion of housing 12 as will be described. Housing 12 has a fitting 50 threaded onto outer tube 23. A cylinder 51 is threaded to fitting 50. A washer 52 is press fit and thereby fixed in fitting 50 and inner tube 21. Inner tube 21 has an upper end which may be fixed relative to or slidable in washer 52, as desired. However, preferably the external surface of inner tube 21 at its upper end fits contiguous or in contact with the surface of washer 52 defining the hole therethrough. A shield 53 made of a magnetic material may be fixed around fitting 50 by one or two or more screws 54. Outer tube 23 has a radial hole 55 therethrough through which the leads from coil 24 pass. Fitting 50 has a hole 56 therethrough in alignment with hole 55 through which the leads from coil 24 pass. From the outward radial extremity of hole 56, the coil leads indicated at 57 and 58 pass upwardly between cylinders 51 and shield 53 and are connected to pins 59 and 60 of the electrical connector 14. Electrical connector 14 may be a conventional five pin connector.

As stated previously, the leads 31 and 32 from crystal 30 extend upwardly through the interior of inner tube 21. At the upper end of inner tube 21, as shown in FIG. 2, leads 31 and 32 are connected to the input of differential amplifier 61. Leads 31 and 32 thus extend outwardly through the upper opening in inner tube 21.

Differential amplifier 61 may be entirely conventional, and mounted on a conventional card, if desired. Amplifier 61 may be supported inside shield 53 by any conventional means, if desired, or simply supported by the strength of leads 31 and 32, and output leads 62 and 63 which are connected to pins 64 and 65 of connector 14, respectively. A lead 66 provides a ground connection from shield 53 to the fifth pin 67 of connector 14.

The manner in which connector 14 is mounted on cylinder 51 is shown in FIG. 6. Only one bolt 15 is shown in FIG. 6 since all bolts 15 are similarly situated. In FIG. 6, bolt 15 is shown having a head 68, a washer 69 under head 68, an O-ring 70 under washer 69, and a shank 71 threaded into cylinder 51. A second O-ring 72 also extends around screw shank 71. O- ring 70 fits between the lower surface of washer 69 and a counter sunk frusto-conical hole 73 in connector 14. O-ring 72 fits between the upper surface of cylinder 51 and another counter sunk frusto-conical hole 74 in connector 14. Holes 73 and 74 are connected by a bore 75. From FIG. 6, it will be noted that all the structures shown therein may vibrate, but that the amount of vibration transmitted to connector 14 may be quite small.

One embodiment of the densitometer of the present invention is illustrated in FIG. 7. Probe 10 is again so indicated as housing driver coil 24, crystal 30, and differential amplifier 61. Crystal 30 is labeled detector in FIG. 7.

A current-to-voltage converter 76 is connected from amplifier 61. Converter 76 is shown in FIG. 8, and may be entirely conventional. In FIG. 8, an amplifier is indicated at 77 having a feedback resistor 78 connected from its output at 79 to its input at 80, amplifier 77 having a ground connection at 81.

In FIG. 7, a differentiator 82 is connected from converter 76 to a squarer 83. Differentiator 82 may likewise be entirely conventional, as shown in FIG. 9.

In FIG. 9, an amplifier 84 has a feedback resistor 85 connected from an output 86 to an input 87. The input to the differentiator 82 is then supplied through a capacitor 88 connected to the amplifier input 87. Amplifier 84 is also supplied with a ground connection 89.

The input to differentiator 82 is mainly a sine wave voltage having a frequency which is equal to the resonant frequency detected by crystal 30. As is conventional, differentiator 82 then produces a sine wave voltage shifted in phase by 90 from the input sine wave voltage and proportional in amplitude to the absolute value of the input frequency. The output sine wave voltage of the differentiator 82 is then converted to a square wave by squarer 83. Since the units of time are not equal to the units of potential, the phrase square wave is, therefore, defined for use herein and in the claims to mean a voltage wave which abruptly arises to a maximum value and stays constant over half the period thereof, and then abruptly drops, for example, with an almost infinite slope again to its minimum value. The square wave then remains at its minimum value for half of its period. Thus, a square wave may have any maximum amplitude and any minimum amplitude without regard to its period or frequency.

An amplitude control 90, a tracking filter 91, and a power amplifier 92 are successively connected from squarer 83 to driver coil 84. A phase comparator 93 receives one input from the output of control 90, another input from the output of filter 91, and supplies an input to a filter frequency control 94. The output of the control 94 is employed to vary electrically the frequency location of the passband of filter 91 to where the signal having the fundamental frequency of the square wave output of control 90 to pass through filter 91 to its output with the least attenuation.

Amplitude control 90 may simply be a voltage divider to reduce the amplitude of the output signal of squarer 83 to a desired value. Note that if all of the blocks of the system of FIG. 7 previously described operate as an electromagnetic oscillator, the oscillation amplitude may increase to infinity at which or before which some of the component parts may fail. Thus, to put a finite limit on the amount of feedback to driver coil 24, control 90 is provided.

Power amplifier 92 produces an alternating output voltage whose average amplitude is somewhat above or below zero. That is, it has a conventional D.C. bias as explained in many publications including, but not limited to, Magnetostriction Transducers, published by the International Nickel Company, Inc., 67 Wall Street, New York, New York 10005. See also, for example, the bibliography of this one publication. The D.C. bias is employed to keep the current flow through driver coil 24 in one direction only and to keep the frequency of the output voltage of crystal 30 equal to that of the input voltage to the driver coil 24.

Phase comparator 93 is entirely conventional. Control 94 and filter 91 are shown in FIG. 10. Filter 91 includes an amplifier 95, a resistor 96 connected from the output to the input of amplifier 95, a capacitor 97 connected from junction 98 to the output of the amplifier 95, a capacitor 99 connected from the junction 98 to the input of amplifier 95, and a resistor 100 connected from the output of control 90 to junction 98.

Control 94 includes an amplifier 101 connected from the output of phase comparator 93 through a resistor 102. A resistor 103 is connected from the output to the input of amplifier 101. Similarly, a capacitor 104 is connected from the output to the input of amplifier 101. The output of amplifier 101 is connected to the gate 105 of a field effect transistor 106. The drain 107 of transistor 106 is connected to junction 98. The source 108 of transistor 106 is connected to ground.

The output of filter 91 is taken at junction 88 and applied both to amplifier 92 and comparator 93.

In FIG. 7, the output of filter 91 is impressed upon a linearization circuit 109. The output of circuit 109 is impressed upon an indicator 110, which may be a voltmeter as shown in FIG. 11. voltmeter 110 may be a D.C. voltmeter linearly calibrated in density.

If desired, so that phase comparator 93 may receive a stronger input signal, the output of a squarer 83 may be connected to comparator 93, and the input thereto from the output of amplitude control 90 may be omitted. Similarly, the connection between the output of filter 91 and comparator 93 may be omitted and squarer may be connected form the output of filter 91 to the right-hand input of comparator 93, as shown in FIG. 7. If the squarer is inserted the linearization circuit 109 may also receive its output.

Linearization circuit 109 is shown in greater detail in FIG. 11 including divide by two circuits 111 and 112, both of which may be identical. Each divide by two circuit may be entirely conventional. For example, divide by two circuit 111 may be simply a binary digital counter adapted for automatic reset on a predetermined count. The output of this counter would then be taken from the last stage thereof.

In general, circuit 111 will produce a square wave output from a positive maximum to ground. The square wave would thus never drop below ground. A relatively large capacitor 113 centers the square wave about ground so that the square wave reaches an approximately maximum value of and a minimum value of E,. As the frequency of the input signal of circuit 111 is f the frequency of the output signal of circuit 111 is f. Thus, f K f. K may be any number larger than zero. However, in the specific case of circuit 111, K 2.

A resistor 114 is connected from capacitor 113 to the input of an amplifier 115. A capacitor 116 is connected from the output to the input of amplifier 115. The same is true of a switch 117. Switches are also provided at 118, 119 and 120. All of the switches 117, 118, 119 and 120 are preferably electrical switches and thus, for example, incorporate transistors. Switches 117, 118, 119 and 120 are entirely conventional.

Switches 117, 118, 119 and are closed for alternate groups of periods. All the periods of one group are equal in time. All the periods in the other group are likewise equal in time. Further, each period of one group is equal to the common period of the other group. Switches 117, 118, 119 and 120 are operated synchronously. That is, all change position at the same time. However, switch 117 may be open or closed when any of the other switches 118, 119 and 120 are open or closed. The same is true of switch 120. Switch 118 may be open or closed regardless of the open or closed states of switches 117 and 120. However, switch 118 must be open when switch 119 is closed, and vice versa. The same is true of switch 119.

A resistor 121 connects the output of amplifier 115 to an amplifier 122. A resistor 123 is connected from the output of amplifier 122 to the input thereof. The same is true of a resistor 124. Switches 118 and 119 are connected on the output of amplifier 122 to ground. Switches 118 and 119 are connected by a mutual junction 125. A capacitor 126 which performs the same function as capacitor 113 and is also relatively large is connected from junction 125. A resistor 127 is connected from capacitor 126 to the input of an amplifier 128. A capacitor 129 is connected from the output to the input of amplifier 128. The same is true of switch 120.

The output of amplifier 128 is connected to the input of amplifier 130 by a variable resistor R A variable resistor R is connected from a source of potential E to the input of amplifier 130. A resistor 131 is connected from the output of amplifier 130 to the input thereof. The same is true of a capacitor 132. The output of amplifier 130 is connected to the input of voltmeter 110. The output of amplifier 130 is a D.C. voltage directly proportional to f2)+ B where A and B are constants. From the previous equation f K f, it is also to be noted that by a mere change in constants, the output voltage of amplifier 130 is directly proportional to o/f0 B where A is also a constant.

As will be explained, the signal appearing at the left end of resistor 127, as viewed in FIG. 11, will be a square wave having a maximum amplitude of +15. and a minimum amplitude of E,,.

In FIG. 10, control 94 may be entirely conventional except for transistor 106. Transistor 106, by itself, may be conventional, but not in the circuit combination or as used. Transistor 106 changes the resistance between junction 98 and ground in accordance with the output of comparator 93. All the structure of control 94, except transistor 106, is simply a D.C. amplifier and filter. Any conventional D.C. amplifier and filter may be substituted therefor.

Whether or not transistor 106 is considered in the circuit of control 94 or in the circuit of filter 91, the major function of the filter 91 is to provide a passband with a tracking control through the connection between transistor 106 and junction 98 via source 107.

Any conventional tracking filter and conventional control, therefore, may be substituted for filter 91 and control 94. Methods other than phase comparison may also be employed to provide an input to the tracking filter frequency control.

In FIG. 11, resistor 114, capacitor 116, and amplifier 115 form a conventional integrator. Any conventional integrator may be substituted therefor. The same is true of resistor 127, capacitor 129 and amplifier 128.

Resistor 121 with resistor 123, capacitor 124 and amplifier 122 form a conventional averaging circuit. The output of amplifier 122 will be a substantially constant D.C. voltage directly proportional to the average value of the voltage appearing at the output of amplifier 115. The same is true of resistor R resistor 131, capacitor 132 and amplifier 130. The fact that resistor R A is variable provides an adjustment feature in accordance with the device of the present invention. The same is true of resistor R That is, the output voltage of amplifier 130 will be directly proportional to the average of the output voltage of amplifier 128, as modified by any adjustments in resistors R A and R For reference, the said integrators are indicated at 133 and 134. The averaging circuits are indicated at 135 and 136.

The input to integrator 133 is a square wave centered about zero volts. The output of integrator 133 would then be a series of triangular waves. Circuit 122 shunts the output of amplifier 115 to its input alternate triangles. Thus, the triangles are spaced between signal values very close to zero volts. Circuit 135 then averages the triangles. Thus, the output of amplifier 122 is a substantially constant DC. voltage directly proportional to the peak amplitudes of the triangles. Since the peak amplitudes are reached after an integration of a square wave, the peak amplitudes are thus directly proportional to period, and inversely proportional to frequency. Hence, the output of circuit 135 is directly proportional to the reciprocal of frequency. Circuit 112 operates switches 118 and 119 to convert the output of circuit 135 again to a square wave having a maximum amplitude equal to the output signal amplitude of circuit 135. This square wave is zero adjusted through capacitor 126 so that, as stated previously, the constant maximum is +12 and the constant minimum is E lntegrator 134 then produces triangular wave, as before, and circuit 112 operating switch 120 cancels out alternate triangles. Averaging circuit 136 then produces an output which is again directly proportional to the triangle peaks. Since the triangle peaks are proportional to the product of E and the triangle period, and E is proportional to the triangle period, the output of circuit 136 is proportional to the period squared or inversely proportional to the frequency squared. Thus, the output of circuit 136 is proportional to It is an outstanding feature of the device of the present invention that a formula may be developed which the output of amplifier 130 follows very closely. Calibration is thus made very easy. The development of this formula follows.

First, assume a container filled with a liquid and suspended from a fixed structure by a coiled spring. 1f the container is pulled downwardly, as is well known, the system will vibrate, i.e. move up and down in yo-yo fashion, at a frequency, f, given by By definition, mass is equal to the product of density and volume. If the fluid has a density, d, and a volume, v, from (5'),

Thus,

Since K. m and v are all constants, we can define constants, A and B, in (8) and (9), respectively, as follows: i

Combining (7). (8) and (9),

In accordance with the device of the present invention, it is striking that an output can be reduced from amplifier that will be directly proportional to d as defined in (10) within a very small accuracy tolerance of :01 percent over a wide range from about 0.08 pounds per cubic foot to about 80.0

pounds per cubic foot. d then becomes the density of the liquid or gas under test, f is one of the frequencies at which vane 20 resonates, preferably the lowest or first resonant frequency. As stated previously, A and B are constants.

The unique character of the invention which causes the output of amplifier 130 to follow d makes it possible to calibrate the densitometer of FIG. 7 very easily, quickly and accurately.

The first step in calibration is to immerse probe 10 from entirely below fitting 17 in a first fluid of known density d,,, and measure the resonant frequency f,,. The second step is to immerse probe 10 entirely below fitting 17 in a second fluid of known density 11,, and measure the resonant frequency, f,,, where d,, is not equal to d That is, the second fluid should not be the same fluid as the first fluid.

After the said calibration steps have been performed, the desired constants, A and B, may then be calculated from the following simultaneous equations (1 l) and (I2), i.e. two equations and two unknowns.

[in d) (l3) dim dam m= C /fH' (20) where E is the DC. output voltage of amplifier 130, and C is a constant of proportionality.

Notice the bracket terms have density units, MIL, and E has units of a difference of potential, e, where M mass, and L length. Thus, C has units eL lM. For example, C may be in units of volt cubic feet for pound.

Stated another way, E is directly proportional to the bracketed terms of (20), and the constant of proportionality, C, is simply determined by whatever arbitrary voltage settings or amplifier gains that are selected for densitometer operation.

OPERATION In the operation of the densitometer shown in FIG. 7, ambient noise will cause detector 30 to pick up signals in a band of frequencies including the resonant frequency of the electromagnetic oscillator. That is, signals will be amplified by amplifier 61, converted from a current to a voltage by converter 76, and differentiated by differentiator 82. The output of differentiator 82 will thus be a sine wave which is converted into a square wave 83. Amplitude control 90 may be used to reduce the output of squarer 83 to a limiting value. The frequency location of the pas-band of tracking filter 91 will then be varied by filter frequency control 94 to follow or pass the fundamental frequency of the output of control 90 to power amplifier 92 with a minimum attenuation. The frequency location of the passband of tracking filter 91 thus will be controlled through varying the resistance of transistor 106, shown in FIG. 10. This will be done in accordance with the differentiation in phases of the output signals of control 90 and filter 91 by phase comparator 93. Power amplifier 92 will then drive coil 24 with a signal in phase with the resonant frequency signal output of detector 30. The vibration produced by coil 24 will the increase in amplitude until limited by amplitude control 90. At this time, the amplitude of the vibration will reach an approximately quiescent level. Should fluid be flowing in pipeline l9, and should the density of the fluid change, the frequency of the output signal of tracking filter 91 will also change. Linearization circuit 109 will then produce a DC. output voltage directly proportional to density. Indicator or voltmeter 110 may then be read, when calibrated in density.

Note will be taken that parts of the probe are vibrated because the alternating signal applied to coil 24, as shown in FIG. 2, will place varying amounts of radial compression on half cylinder 26, inner tube 21 expanding and contracting axially between shank 40 and the abutment of inner tube 21 at its lower end with its half cylinder 26.

It is an advantage of the device of the present invention that differentiator amplifier 61 and converter 76 is used. For example, converter 76 may be located a great distance from probe 10. Converter 76 has a low resistance input and is thus a ground input device. The accuracy by which the resonant frequency signal output of detector 30 is transmitted to converter 76 is not affected by longitudinal transmission line between probe 10 and converter 76. That is, the ground magnitude is accurate regardless of the length of line. The voltage drop along the line, therefore, does not affect the accuracy when the resonant frequency signal is transmitted to converter 76 from differentiator amplifier 61. The rejection of external noise is also to be limited.

It is a feature of the invention that the interference fit of sleeve 22 on half cylinders 26 and 27 is employed to improve efficiency and accuracy. The use of crystal 30 also makes it possible to employ a vibrating structure which is very small. The continuous compression fit of projection 39 between shank 43 and the upper side of half cylinder 26, as shown in FIG. 2, also improves vibration efficiency. The operation of the leads from crystal 30 from the leads from coil 24 is also a substantial advantage. The current through coil 24 thus cannot induce a feedback current in the crystal leads. Note that the crystal leads and the coil leads are entirely separated and notice that they extend above the top of shield 53 in FIG. 2. As stated previously, shield 53 is magnetic. So is fitting 50; so is washer 52; so is inner tube 21. The crystal leads are thus completely enclosed in a magnetic shield until they reach amplifier 61. Note, too, that shield 53 seals everything inside thereof from coil leads 57 and 58 until they reach the top of shield 53. The output of amplifier 61, the induction from amplifier 61 has raised the level of the input signal thereto to a substantial value. No substantial unwanted induction thus takes place between the coil leads and the output leads of amplifier leads 61 above the top of shield 53 which, as stated previously, is made of magnetic material.

It is also an advantage of the invention that O-rings 46 and 47 resiliently mount probe shank 11 through fitting 17. Substantially improved efficiency and accuracy are to be achieved. The same is true of the resilient mount of electrical connector 14 through O-rings 70 and 72.

One outstanding advantage of the present invention resides in the use of differentiator 82 with tracking filter 91. Differentiator 82 acts, more or less, as a high pass filter. If the amplitude of output signals of differentiator 82 are plotted as a function of frequency, this amplitude would be substantially a straight line of a predetermined positive slope, amplitude being indicated positive vertically upward on the ordinant, and frequency being indicated positive to the right. In spite of the fact that differentiator 82 acts as a very good high pass filter, it provides a constant .90 degree phase shift of input signals thereto. Tracking filter 91 conveniently also provides such a phase shift at the frequency at which signals are attenuated the least. Differentiator 82 and tracking filter 91 provide a phase shift in the same direction, i.e. lead or lag. This means that the output of power amplifier 92 is an alternating signal which may be adjusted in phase, i.e. 180 or zero, simply by reversing the leads 57 and 58 from driver coil 24 connected therefrom. An in-phase drive is to be effected.

Another feature of the invention uses the double integration system to derive the function An outstanding advantage of the invention resides in the use of switches 117, 118, 119 and 120 with circuit 112 to prevent any frequency modulation of the input to integrator 133 from destroying the accuracy of the output of circuit 136.

The function of the double integration is easily understood when it is considered that the integral of E, a constant, with respect to X, results in EX, and the integral of EX results in EX /2.

Note will be taken that an outstanding feature of the invention resides in the use of probe 10. Probe 10 may be used with any pipe size and has substantial sensitivity and accuracy. No pressure differentiators or changes are involved. The densitometer of the present invention may have an accuracy tolerance of 0.01 percent over a range of, for example, 0.08 pounds per cubic foot to 80.0 pounds per cubic foot. Vane 20 is small and compact and does not disturb the flow of pipeline 19. The same is true of other structures surrounding vane 20.

Note at the bottom of FIG. 2, projection 39, outer tube 23, boss 36, and inner tube 21 form a completely closed magnetic circuit about driver coil 24 for maximum driving accuracy. Note, too, that inner tube 21 is nowhere cemented or phased by bonding or the like to outer tube 23, inner tube 21 only being fixed in outer tube 23 magnetically. The same is true of the bottom inner tube 21, and boss 36 and sleeve 22. That is, inner tube 21 is not bonded or otherwise similarly fixed to boss 36 or sleeve 22, but is only, more or less, slidable therethrough. What is meant by not fixed" in the immediately preceding description is that, further, there is no press fit. Similarly, the inner end of inner tube 21 is not fixed to half cylinder 26.

Note will be taken that one feature of the invention may be used without any of the other features. Moreover, any one feature may be used with any one or more or all of the other features. The invention, therefore, is not to be limited by any one feature.

What is claimed is:

l. A vibration densitometer comprising: a probe having a structure adapted to be vibrated at one end thereof, said structure including a main cylinder and a vane in the shape of a substantially flat plate of uniform thickness, said vane passing approximately through the axis of said main cylinder from one side thereof to the other, first means to clamp both of the edges of said vane contiguous to the internal surface of said main cylinder, said first means being fixed relative to said main cylinder, the internal surface of said first means being substantially cylindrical and concentric with that of said main cylinder, said main cylinder having a substantially uniform thickness around its circumference, second means to vibrate said first means at a point approximately midway along its length, and at a point spaced about 90 mechanical degrees around said main cylinder from one longitudinal edge of said vane, third means to sense vibrations of said vane and fourth means connected from said third means to produce an output signal in accordance with the density of fluid surrounding said vane.

2. The invention as defined in claim 1, wherein said probe includes an outer cylinder having an axis approximately perpendicular to that of said main cylinder and extending approximately through said main cylinder axis approximately at its midpoint, one end of said outer cylinder being fixed relative to said main cylinder, a magnetostrictive inner tube inside said outer tube, said outer tube having an inwardly extending annular shoulder, said inner tube having an outwardly extending annular shoulder between said first means and said outer tube shoulder in engagement with said outer tube shoulder, said inner tube having one end in engagement with said first means, a solenoid winding around said inner tube inside said outer tube fixed to said inner tube, said winding being located between said first means and said outer tube shoulder, said inner tube always being in axial compression between said first means and said outer tube shoulder whether or not said winding is energized, fifth means between the ends of said inner and outer tubes, said inner and outer tubes and said fifth means all being made of a magnetic material and being located contiguous to each other to form a substantially closed magnetic circuit around said winding, a piezoelectric crystal fixed to said vane approximately at the center of one longitudinal edge thereof, said crystal and said winding each having a pair of lead wires, said crystal lead wires extending through the center of said inner tube, said winding lead wires extending in the space between said inner and outer tubes, sixth means to energize said winding with a signal in phase with the output of said crystal at the first resonant frequency, seventh means responsive to the output of said crystal for producing an output signal directly proportional to lf) B where,

f is the first resonant frequency, and

A and B are constants, and eight means including D.C. voltmeter calibrated linearly in density to indicate the magnitude of said seventh means output signal.

3. A vibration densitometer comprising: a probe having a structure adapted to be vibrated at one end thereof, said structure being in the shape of a thin walled hollow cylinder, and a vane fixed longitudinally inside said cylinder, said vane extending approximately through the axis of said cylinder, said cylinder wall having a thickness small in comparison to its diameter, said vane having its longitudinal edges fixed relative to said cylinder and being otherwise unsupported between said edges, said vane being adapted to vibrate over the unsupported portion thereof, means to vibrate said vane, means to sense vibrations of said vane, and means connected from said sensing means to produce an output signal in accordance with the density of fluid surrounding said vane.

4. The invention as defined in claim 3, including two half cylinders inside said hollow cylinder to clamp the axially extending edges of said vane, said vibrating means comprising means to vary the strain of one of said half cylinders by applying an undulating force at a point approximately midway along the length of the external surface thereof and approximately midway therearound.

5. The invention as defined in claim 3, wherein both circular edges of said cylinder at the ends thereof are rounded.

6. A vibration densitometer comprising: a probe having a structure adapted to be vibrated at one end thereof, said probe including inner and outer hollow tubes, said structure being fixed relative to the end of said outer tube, said tubes both being made of magnetic materials, a detector for producing an electrical signal in accordance with the vibration of said structure, a solenoid winding between said tubes around said inner tube, said inner tube being magnetostrictive, said detector having a pair of lead wires extending from said one probe end through the hollow interior of said inner tube, said winding having a pair of leads extending away from said one probe end in the space between said tubes, first means to provide a magnetic circuit surrounding the end of said inner tube at the other end of said probe, said first means defining an enclosed housing space, an electrical circuit in said housing space, said detector lead wires being connected to said circuit, said first means providing a completely closed magnetic circuit except for one hole therethrough, said circuit having output leads extending through said hole, said inner tube being adapted to move and vibrate said structure responsive to the flow of a fluctuating current in said solenoid winding.

7. A vibration densitometer comprising: a probe having a structure adapted to be vibrated at one end thereof, said structure being a substantially rectangular vane, first means to clamp the opposite longitudinal edges of said vane, said probe including a longitudinal member, said structure and said first means being mounted on said member, second means to vibrate said vane, third means to sense vibrations of said vane, and fourth means connected from said third means to produce an output signal in accordance with the density of fluid surrounding said vane.

8. The invention as defined in claim 7, wherein said first means includes two half cylinders and a ring around said half cylinders, said ring and half cylinders with said vane therebetween having an interference fit.

9. The invention as in claim 8, wherein said third means comprises a detector positioned against one clamped edge of said vane, and said second means vibrates one half cylinder at a point therearound approximately about the axis of said ring from said detector.

10. The invention as defined in claim 9, wherein said second means includes third means responsive to an electrical signal to vibrate said one half cylinder, fourth means for impressing an alternating signal on said third means in phase with the output of said detector, said fourth means being connected from said detector.

11. The invention as defined in claim 10, including fifth means connected from said detector to produce an output signal which is a function of the frequency of the output signal of said detector.

12. The invention as defined in claim 11, wherein said detector is a device to produce an electrical signal in accordance with pressure changes thereon.

13. The invention as defined in claim 12, wherein said detector is a piezoelectric crystal.

14. The invention as defined in claim 13, wherein said crystal is disposed midway along the length of said vane.

15. The invention as defined in claim 7, including a pipe,

said pipe having a hole through the wall thereof, said probe longitudinal member extending through said hole, said vane being located inside said pipe substantial in a plane parallel to the pipe axis but perpendicular to the axis of said member, second means to provide a fluid tight seal between said member and the edges of said pipe hole.

16. The invention as defined in claim 15, wherein said second means resiliently mounts said probe in said pipe hole.

17. The invention as defined in claim 16, wherein said member has an outer cylindrical surface, said second means including two O-rings, said second means being everywhere spaced from the said outer surface of said member except that each of said O-rings lie in fluid tight sealing and supporting contact with the outer surface of said member at two respective spaced circular lines of contact therewith.

18. A vibration densitometer comprising: a probe including a body; a hollow outer cylinder fixed relative to said body; a hollow inner cylinder having at least an approximately cylindrical internal bore and bore axis, said inner cylinder being fixed relative to and contiguous to said outer cylinder inside thereof and concentric therewith; a substantially flat, rectangular vane having a substantially uniform thickness between approximately parallel planes, the thickness of said vane being small in comparison to the length and width thereof, said vane having its longitudinal edges supported in fixed positions relative to said inner cylinder inside thereof, said vane extending generally diametrically across said inner cylinder bore through said bore axis, said outer cylinder having a hole extending therethrough in a predetermined direction approximately perpendicular to said planes; a piezoelectric crystal fixed between said cylinders against one of said vane edges approximately at the midpoint thereof; a magnetostrictive member, said member extending in said predetermined direction from a first position spaced from the external cylindrical surface of said outer cylinder a distance from said bore axis greater than the radius of said cylinder external surface, said member extending from said first position loosely through said hole to a second position at a predetermined portion of the external cylindrical surface of said inner cylinder accessible through said hole, said member having one end at said second position in contact with said surface portion; loading means fixed relative to said body and engaging said member at said first position, said loading means holding said member in axial compression with said end thereof in pressure contact with said surface portion, a winding around said member between said first and second positions; utilization means having an input; first means connecting said crystal to said utilization means input, said first means having an output connected to said utilization means input; and second means connected from said first means output to said winding to impress a predetermined voltage thereon having at least an AC. component with a predetermined phase and predetermined frequency the same as those of the output voltage of said crystal, the application of said predetermined voltage to said winding causing the strain in said member, said inner cylinder, said outer cylinder and said vane to vary said vane vibrating in accordance with said varying strain, said crystal detecting said vane vibration, said vane, said crystal, said first means, said second means, said winding and said member acting as a closed loop electromechanical oscillator with the feedback signal being produced by said crystal, at least one of said first and second means including an amplifier, the gain of said loop being adequate to sustain vibration of said vane continuously.

19. The invention as defined in claim 18, wherein said cylinders have an interference fit.

20. The invention as defined in claim 18, wherein a hollow outer tube is fixed relative to said body in a position surrounding said magnetostrictive member and said winding, said outer tube being spaced from said member to accommodate said winding over the length thereof.

21. The invention as defined in claim 20, wherein said outer tube is positioned contiguous to said member at each end of said winding, said outer tube being made of a magnetic material to complete a magnetic circuit from two points on said member beyond each end of said winding.

22. The invention as defined in claim 21, wherein one end of said outer tube is fixed relative to and sealed to the external cylindrical surface of said outer cylinder around said hole therethrough.

23. The invention as defined in claim 22, wherein said magnetostrictive member is a hollow inner tube, said crystal having a pair of leads extending from between said cylinders through the interior of said inner tube to said first means.

24. The invention as defined in claim 21, wherein said magnetostrictive member is a hollow inner tube, said crystal having a pair of leads extending from between said cylinders through the interior of said inner tube to said first means.

25. The invention as defined in claim 21, wherein one end of said outer tube is fixed relative to said outer cylinder around said hole therethrough.

26. The invention as defined in claim 20, wherein one end of said outer tube is fixed relative to and sealed to the external cylindrical surface of said outer cylinder around said hole therethrough.

27. The invention as defined in claim 26, wherein said magnetostrictive member is a hollow inner tube, said crystal having a pair of leads extending from between said cylinders through the interior of said inner tube to said first means.

28. The invention as defined in claim 20, wherein said magnetostrictive member is a hollow inner tube, said crystal having a pair of leads extending from between said cylinders through the interior of said inner tube to said first means.

29. The invention as defined in claim 28, wherein one end of said outer tube is fixed relative to said outer cylinder around said hole therethrough.

30. The invention as defined in claim 29, wherein said outer tube is positioned contiguous to said member at each end of said winding, said outer tube being made of a magnetic material to complete a magnetic circuit from two points on said member beyond each end of said winding. 

1. A vibration densitometer comprising: a probe having a structure adapted to be vibrated at one end thereof, said structure including a main cylinder and a vane in the shape of a substantially flat plate of uniform thickness, said vane passing approximately through the axis of said main cylinder from one side thereof to the other, first means to clamp both of the edges of said vane contiguous to the internal surface of said main cylinder, said first means being fixed relative to said main cylinder, the internal surface of said first means being substantially cylindrical and concentric with that of said main cylinder, said main cylinder having a substantially uniform thickness around its circumference, second means to vibrate said first means at a point approximately midway along its length, and at a point spaced about 90 mechanical degrees around said main cylinder from one longitudinal edge of said vane, third means to sense vibrations of said vane and fourth means connected from said third means to produce an output signal in accordance with the density of fluid surrounding said vane.
 2. The invention as defined in claim 1, wherein said probe includes an outer cylinder having an axis approximately perpendicular to that of said main cylinder and extending approximately through said main cylinder axis approximately at its midpoint, one end of said outer cylinder being fixed relative to said main cylinder, a magnetostrictive inner tube inside said outer tube, said outer tube having an inwardly extending annular shoulder, said inner tube having an outwardly extending annular shoulder between said first means and said outer tube shoulder in engagement with said outer tube shoulder, said inner tube having one end in engagement with said first means, a solenoid winding around said inner tube inside said outer tube fixed to said inner tube, said winding being located between said first means and said outer tube shoulder, said inner tube always being in axial compression between said first means and said outer tube shoulder whether or not said winding is energized, fifth means between the ends of said inner and outer tubes, said inner and outer tubes and said fifth means all being made of a magnetic material and being located contiguous to each other to form a substantially closed magnetic circuit around said winding, a piezoelectric crystal fixed to said vane approximately at the center of one longitudinal edge thereof, said crystal and said winding each having a pair of lead wires, said crystal lead wires extending through the center of said inner tube, said winding lead wires extending in the space between said inner and outer tubes, sixth means to energize said winding with a signal in phase with the output of said crystal at the first resonant frequency, seventh means responsive to the output of said crystal for producing an output signal directly proportional to (A/f2) + B where, f is the first resonant frequency, and A and B are constants, and eight means including D.C. voltmeter calibrated linearly in density to indicate the magnitude of said seventh means output signal.
 3. A vibration densitometer comprising: a probe having a structure adapted to be vibrated at one end thereof, said structure being in the shape of a thin walled hollow cylinder, and a vane fixed longitudinally inside said cylinder, said vane extending approximately through the axis of said cylinder, said cylinder wall having a thickness small in comparison to its diameter, said vane having its longitudinal edges fixed relative to said cylinder and being otherwise unsupported between said edges, said vane being adapted to vibrate over the unsupported portion thereof, means to vibrate said vane, means to sense vibrations of said vane, and means connected from said sensing means to produce an output signal in accordance with the density of fluid surrounding said vane.
 4. The invention as defined in claim 3, including two half cylinders inside said hollow cylinder to clamp the axially extending edges of said vane, said vibrating means comprising means to vary the strain of one of said half cylinders by applying an undulating force at a point approximately midway along the length of the external surface thereof and approximately midway therearound.
 5. The invention as defined in claim 3, wherein both circular edges of said cylinder at the ends thereof are rounded.
 6. A vibration densitometer comprising: a probe having a structure adapted to be vibrated at one end thereof, said probe including inner and outer hollow tubes, said structure being fixed relative to the end of said outer tube, said tubes both being made of magnetic materials, a detector for producing an electrical signal in accordance with the vibration of said structure, a solenoid winding between said tubes around said inner tube, said inner tube being magnetostrictive, said detector having a pair of lead wires extending from said one probe end through the hollow interior of said inner tube, said winding having a pair of leads extending away from said one probe end in the space between said tubes, first means to provide a magnetic circuit surrounding the end of said inner tube at the other end of said probe, said first means defining an enclosed housing space, an electrical circuit in said housing space, said detector lead wires being connected to said circuit, said first means providing a completely closed magnetic circuit except for one hole therethrough, said circuit having output leads extending through said hole, said inner tube being adapted to move and vibrate said structure responsive to the flow of a fluctuating current in said solenoid winding.
 7. A vibration densitometer comprising: a probe having a structure adapted to be vibrated at one end thereof, said structure being a substantially rectangular vane, first means to clamp the opposite longitudinal edges of said vane, said probe including a longitudinal member, said structure and said first means being mounted on said member, second means to vibrate said vane, third means to sense vibrations of said vane, and fourth means connected from said third means to produce an output signal in accordance with the densiTy of fluid surrounding said vane.
 8. The invention as defined in claim 7, wherein said first means includes two half cylinders and a ring around said half cylinders, said ring and half cylinders with said vane therebetween having an interference fit.
 9. The invention as in claim 8, wherein said third means comprises a detector positioned against one clamped edge of said vane, and said second means vibrates one half cylinder at a point therearound approximately 90* about the axis of said ring from said detector.
 10. The invention as defined in claim 9, wherein said second means includes third means responsive to an electrical signal to vibrate said one half cylinder, fourth means for impressing an alternating signal on said third means in phase with the output of said detector, said fourth means being connected from said detector.
 11. The invention as defined in claim 10, including fifth means connected from said detector to produce an output signal which is a function of the frequency of the output signal of said detector.
 12. The invention as defined in claim 11, wherein said detector is a device to produce an electrical signal in accordance with pressure changes thereon.
 13. The invention as defined in claim 12, wherein said detector is a piezoelectric crystal.
 14. The invention as defined in claim 13, wherein said crystal is disposed midway along the length of said vane.
 15. The invention as defined in claim 7, including a pipe, said pipe having a hole through the wall thereof, said probe longitudinal member extending through said hole, said vane being located inside said pipe substantial in a plane parallel to the pipe axis but perpendicular to the axis of said member, second means to provide a fluid tight seal between said member and the edges of said pipe hole.
 16. The invention as defined in claim 15, wherein said second means resiliently mounts said probe in said pipe hole.
 17. The invention as defined in claim 16, wherein said member has an outer cylindrical surface, said second means including two O-rings, said second means being everywhere spaced from the said outer surface of said member except that each of said O-rings lie in fluid tight sealing and supporting contact with the outer surface of said member at two respective spaced circular lines of contact therewith.
 18. A vibration densitometer comprising: a probe including a body; a hollow outer cylinder fixed relative to said body; a hollow inner cylinder having at least an approximately cylindrical internal bore and bore axis, said inner cylinder being fixed relative to and contiguous to said outer cylinder inside thereof and concentric therewith; a substantially flat, rectangular vane having a substantially uniform thickness between approximately parallel planes, the thickness of said vane being small in comparison to the length and width thereof, said vane having its longitudinal edges supported in fixed positions relative to said inner cylinder inside thereof, said vane extending generally diametrically across said inner cylinder bore through said bore axis, said outer cylinder having a hole extending therethrough in a predetermined direction approximately perpendicular to said planes; a piezoelectric crystal fixed between said cylinders against one of said vane edges approximately at the midpoint thereof; a magnetostrictive member, said member extending in said predetermined direction from a first position spaced from the external cylindrical surface of said outer cylinder a distance from said bore axis greater than the radius of said cylinder external surface, said member extending from said first position loosely through said hole to a second position at a predetermined portion of the external cylindrical surface of said inner cylinder accessible through said hole, said member having one end at said second position in contact with said surface portion; loading means fixed relative to said body and engaging said member at said first position, said loading Means holding said member in axial compression with said end thereof in pressure contact with said surface portion, a winding around said member between said first and second positions; utilization means having an input; first means connecting said crystal to said utilization means input, said first means having an output connected to said utilization means input; and second means connected from said first means output to said winding to impress a predetermined voltage thereon having at least an A.C. component with a predetermined phase and predetermined frequency the same as those of the output voltage of said crystal, the application of said predetermined voltage to said winding causing the strain in said member, said inner cylinder, said outer cylinder and said vane to vary said vane vibrating in accordance with said varying strain, said crystal detecting said vane vibration, said vane, said crystal, said first means, said second means, said winding and said member acting as a closed loop electromechanical oscillator with the feedback signal being produced by said crystal, at least one of said first and second means including an amplifier, the gain of said loop being adequate to sustain vibration of said vane continuously.
 19. The invention as defined in claim 18, wherein said cylinders have an interference fit.
 20. The invention as defined in claim 18, wherein a hollow outer tube is fixed relative to said body in a position surrounding said magnetostrictive member and said winding, said outer tube being spaced from said member to accommodate said winding over the length thereof.
 21. The invention as defined in claim 20, wherein said outer tube is positioned contiguous to said member at each end of said winding, said outer tube being made of a magnetic material to complete a magnetic circuit from two points on said member beyond each end of said winding.
 22. The invention as defined in claim 21, wherein one end of said outer tube is fixed relative to and sealed to the external cylindrical surface of said outer cylinder around said hole therethrough.
 23. The invention as defined in claim 22, wherein said magnetostrictive member is a hollow inner tube, said crystal having a pair of leads extending from between said cylinders through the interior of said inner tube to said first means.
 24. The invention as defined in claim 21, wherein said magnetostrictive member is a hollow inner tube, said crystal having a pair of leads extending from between said cylinders through the interior of said inner tube to said first means.
 25. The invention as defined in claim 21, wherein one end of said outer tube is fixed relative to said outer cylinder around said hole therethrough.
 26. The invention as defined in claim 20, wherein one end of said outer tube is fixed relative to and sealed to the external cylindrical surface of said outer cylinder around said hole therethrough.
 27. The invention as defined in claim 26, wherein said magnetostrictive member is a hollow inner tube, said crystal having a pair of leads extending from between said cylinders through the interior of said inner tube to said first means.
 28. The invention as defined in claim 20, wherein said magnetostrictive member is a hollow inner tube, said crystal having a pair of leads extending from between said cylinders through the interior of said inner tube to said first means.
 29. The invention as defined in claim 28, wherein one end of said outer tube is fixed relative to said outer cylinder around said hole therethrough.
 30. The invention as defined in claim 29, wherein said outer tube is positioned contiguous to said member at each end of said winding, said outer tube being made of a magnetic material to complete a magnetic circuit from two points on said member beyond each end of said winding. 